Lithium disilicate glass-ceramic, method for production thereof and use thereof

ABSTRACT

The invention relates to glass-ceramics based on the lithium disilicate system which can be mechanically machined easily in an intermediate step of crystallisation and, after complete crystallisation, represent a very strong, highly-translucent and chemically-stable glass-ceramic. Likewise, the invention relates to a method for the production of these glass-ceramics. The glass-ceramics according to the invention are used as dental material.

The invention relates to glass-ceramics based on the lithium disilicatesystem which can be mechanically machined easily in an intermediate stepof crystallisation and, after complete crystallisation, represent a verystrong, highly-translucent and chemically-stable glass-ceramic.Likewise, the invention relates to a method for the production of theseglass-ceramics. The glass-ceramics according to the invention are usedas dental material.

Lithium disilicate glass-ceramics are well known from the literature andseveral patents are based on this glass-ceramic system. Thus, forexample, self-glazed lithium disilicate glass-ceramic objects for theproduction of tableware are described in EP-B-536 479, in EP-B-536 572lithium disilicate glass-ceramics which can be used, by scatteringfine-particle coloured glass on the surface thereof, as lining elementsfor building purposes.

The main focus of patented lithium disilicate glass-ceramics is ondental applications. This is due to the fact that the crystallisation oflithium disilicate crystals is effected via a phase of lesser strength(lithium metasilicate) and the material system is consequently amenableinter alia to chair-side methods (see S. D. Stookey: “Chemical Machiningof Photosensitive Glass”, Ind. Eng. Chem., 45, pp. 115-118 (1993) and S.D. Stookey: “Photosensitively Opacifiable Glass” U.S. Pat. No. 2,684,911(1954)). Investigations by Borom, e.g. M. -P Borom, A. M. Turkalo, R. H.Doremus: “Strength and Microstructure in Lithium DisilicateGlass-Ceramics”, J. Am. Ceream Soc., 58, No. 9-10, pp. 385-391 (1975)and M. -P. Borom, A. M. Turkalo, R. H. Doremus: “Verfahren zumHerstellen von Glaskeramiken” (Method for the production ofglass-ceramics), DE-A-24 51 121 (1974) show that glass-ceramics whichcomprise lithium metasilicate as main phase have reduced strength incomparison with glass-ceramics which comprise lithium disilicate assingle crystalline phase.

This principle was used in order firstly to produce a glass-ceramic, ina two-step crystallisation process, which can be machined wellmechanically, e.g. by means of CAD/CAM methods, and to process thissubsequently in a second crystallisation step to form dentalglass-ceramic. This method is suitable for being able to use dentalrestorations according to the so-called chair-side method. In the caseof this method, an individually adapted crown/onlay/inlay is milled outof a glass-ceramic block after the first crystallisation step by meansof CAD/CAM in the dentist's surgery, this is subjected to the secondcrystallisation step in a special oven and used directly in the firstand only dental appointment for the patient (DE 10 2005 028 637).

In addition, in WO-A-95/32678 and U.S. Pat. No. 5,507,981, lithiumdisilicate glass-ceramics were described, which can be processed to formshaped dental products by means of hot-pressing by using a specialcompressible crucible. Furthermore, there are known, from DE-C-14 21886, glass-ceramics based on SiO₂ and Li₂O which contain largequantities of physiologically very questionable arsenic oxide. Also inU.S. Pat. No. 4,515,634 and in FR-A-2 655 264, lithium disilicateglass-ceramics which are suitable for the production of dental crownsand bridges are disclosed.

All known lithium disilicate glass-ceramics display inadequacies in theprocessing thereof to shaped products and/or in mechanical or visualproperties and/or in chemical stability. In particular when used in thedental field, equally high requirements for all the mentioned propertiesmust be fulfilled.

Starting herefrom, it was the object of the present invention to providea glass-ceramic which has improved mechanical and optical properties andalso improved chemical stability relative to the (glass-) ceramics knownfrom the state of the art.

This object is achieved by the lithium disilicate glass-ceramic havingthe features of claim 1 and also by the method for the production ofthis glass-ceramic having the features of claim 9. In claim 12, usesaccording to the invention are indicated. Likewise, a shaped dentalproduct having the features of claim 13 is provided. The furtherdependent claims reveal advantageous developments.

Within the scope of the present invention, glass compositions have beendeveloped which can be prepared in a two-step production process, areeasy to machine after the first crystallisation step, in particular bymeans of CAD/CAM, and, after a very short second crystallisation step,are both highly-transparent and very strong and have better chemicalstabilities than the known lithium disilicate glass-ceramics.

It was shown surprisingly that the addition of ZrO₂ to certain glasscompositions leads to glass-ceramics which can be machined very readilyin an intermediate crystallisation step and, in the end state, haveexcellent strength values, exceptional translucence and significantlyincreased chemical stabilities.

It was shown that up to 20% by weight of a stabiliser selected from thegroup consisting of ZrO₂, HfO₂ or mixtures hereof can be incorporated inthe glass without having a significant influence on the structure.Contrary to all expectations, the stabiliser does not hereby crystalliseout as a separate crystal phase but remains in the remaining glassphase. As a result of the high proportion in the amorphous phase, themechanical and chemical stabilities in this phase are hugely improved,which also leads to improved properties in the end product.

In particular the chemical stability can be improved via the compositionof the remaining glass phase since the glass phase has a significantlyhigher solubility than the lithium disilicate and hence represents theweak point with respect to chemical attack. The extremely highsolubility of the stabiliser (ZrO₂) in the glass phase is in particularremarkable since e.g. zirconium oxide acts in many silicateglass-ceramics as nucleation agent, i.e. crystallises out as first phaseduring a temperature treatment, and the actually sought crystal phase isfacilitated and is deposited in a fine-crystalline manner on these ZrO₂crystals.

As a result of the high proportions of stabiliser which remainessentially in the amorphous phase, the crystalline proportion iscorrespondingly restricted. As a result, and due to the low crystallitesize of the lithium disilicate crystals, good translucence of thematerials is produced after the second crystallisation. The translucenceis however also further improved by the refractive index of the glassphase being increased in turn by the stabiliser and, consequently, beingadapted to the refractive index of the lithium disilicate. In the caseof glass-ceramics in which the refractive index of the amorphous matrixphase corresponds to the refractive index of the crystallinephase/phases, very good translucence properties are found, relativelyirrespective of the crystallite size. In the glass-ceramics according tothe invention, therefore all three points for the production of anextremely translucent glass-ceramic are fulfilled:

-   -   limited crystal phase proportion,    -   small crystals (<500 nm),    -   adapted refractive index of amorphous and crystalline phase.

The high proportion of stabiliser has the effect therefore in theglass-ceramic of

-   -   improved chemical stability,    -   higher strength values and    -   improved translucence in several respects to corresponding        glass-ceramics without or with only a low ZrO₂— or HfO₂        proportion.

The glass-ceramics according to the invention can be produced preferablyby means of a method, in which

a) an initial glass is produced which comprises the components of theglass-ceramic,

b) the initial glass is subjected to a first heat treatment at a firsttemperature in order to produce a glass-ceramic which has lithiummetasilicate as single or main crystal phase and

c) this glass-ceramic is subjected to a second heat treatment in whichthe lithium metasilicate is converted with SiO₂ from the glass phaseinto lithium disilicate and subsequently lithium disilicate is presentas single or main crystal phase.

The crystallisation to form lithium metasilicate preferably takes placeat temperatures between 620° C. and 800° C., with times between 1 and200 minutes, preferably between 650° C. and 750° C. for 10 to 60minutes.

The crystallisation to form lithium disilicate preferably takes place attemperatures between 800° C. and 1,040° C., with times of 5 to 200minutes, preferably between 800° C. and 870° C. for 5 to 30 minutes.

The subject according to the invention is intended to be explained inmore detail with reference to the subsequent examples without wishing torestrict said subject to the special embodiments shown here.

EXAMPLES 1 to 6

In examples 1 to 6, compositions of glasses with a high zirconium oxidecontent are indicated, which are converted by a two-step temperaturetreatment firstly into readily mechanically machinable lithiummetasilicate glass-ceramics and subsequently into highly-translucent,very strong and chemically-stable lithium disilicate glass-ceramics.

The compositions with their components are represented in Table 1.

TABLE 1 B1 B2 B3 B4 B5 B6 SiO₂ 66.9 65.8 65.5 63.7 63.5 63.5 Li₂O 13.913.7 13.6 13.2 14.4 12.9 ZrO₂ 10.0 10.0 12.0 11.7 12.7 13.5 Al₂O₃ 3.23.1 3.1 3.0 3.3 3.5 P₂O₅ 3.0 3.0 3.0 2.9 3.1 3.4 K₂O 2.9 2.9 2.9 2.8 3.03.2 CeO₂ — 1.0 — 2.0 — — Er₂O₃ — 0.2 — 0.3 — — Tb₂O₃ — 0.3 — 0.3 — —

The glasses were melted at 1,500° C. and poured into metal moulds toform blocks. The blocks were stress-relieved at 560° C. in the furnaceand cooled slowly. For the different characterisation processes, theglass blocks were divided up and subjected to a first crystallisationtreatment. For this purpose, the glasses were aged for 10 to 120 minutesat 600° C. to 750° C. As a result, glass-ceramics with strength valuesof 150 MPa to 220 MPa were produced. Exclusively lithium metasilicatewas hereby established as crystal phase. In this state, machining bymeans of CAD/CAM methods is very readily possible.

With a second short crystallisation at 800° C. to 950° C. for 3 to 15minutes, recrystallisation of the lithium metasilicate with amorphousSiO₂ from the glass phase takes place to form lithium disilicate and theresult is an increase in strength to 300 MPa to 450 MPa. In addition tothe lithium disilicate phase, a subsidiary crystal phase with azirconium oxide content can hereby be produced. In addition, also smallresidues of lithium metasilicate can be present. The unequivocal maincrystal phase is lithium disilicate.

In Table 2, the crystallisation conditions of individual glasses andalso the resulting crystal phases and strength values are displayed.

TABLE 2 Glass B1 B2 B3 B4 B5 B6 1. Crystallisation 650° C. 700° C. 650°C. 700° C. 700° C. 700° C. 20 min 40 min 30 min 20 min 40 min 40 min 2.Crystallisation 850° C. 830° C. 870° C. 850° C. 820° C. 830° C. 10 min10 min 20 min 8 min 10 min 10 min Crystal phases Main phase disilicatedisilicate disilicate disilicate disilicate disilicate (>80%) Subsidiaryphase — — — — metasilicate metasilicate (<20%) Translucence excellentvery good excellent very good excellent excellent 3-point 375 MPa 413MPa 380 MPa 418 MPa 356 MPa 385 MPa bending strength

1. A lithium disilicate glass-ceramic having the following composition:55 to 70% by weight of SiO₂, 10 to 15% by weight of Li0₂, 10 to 20% byweight of the stabiliser selected from the group consisting of ZrO₂,HfO₂ and mixtures thereof, 0.1 to 5% by weight of K₂O, 0.1 to 5% byweight of Al₂O₃, 0 to 10% by weight of additives selected from the groupconsisting of boron oxide, phosphorus oxide, fluorine, sodium oxide,barium oxide, strontium oxide, magnesium oxide, zinc oxide, calciumoxide, yttrium oxide, titanium oxide, niobium oxide, tantalum oxide,lanthanum oxide and mixtures thereof, and 0 to 10% by weight ofcolourants.
 2. The lithium disilicate glass-ceramic according to claim1, wherein the colourants are glass-colouring oxides and/or pigments. 3.The lithium disilicate glass-ceramic according to claim 1, wherein theglass-colouring oxides are selected from the group of the oxides ofiron, titanium, cerium, copper, chromium, cobalt, nickel, manganese,selenium, silver, indium, gold, and rare earth metals.
 4. The lithiumdisilicate glass-ceramic according to claim 1, wherein the pigments aredoped spinels.
 5. The lithium disilicate glass-ceramic according toclaim 1 having the following composition: 58 to 64% by weight of SiO₂,11 to 13% by weight of LiO₂, 10 to 15% by weight of the stabiliserselected from the group consisting of ZrO₂, HfO₂ and mixtures thereof, 2to 5% by weight of K₂O, 2 to 5% of Al₂O₃, 2 to 5% of P₂O₅ and also 0 to5% by weight of additives selected from the group consisting of boronoxide, phosphorus oxide, fluorine, sodium oxide, barium oxide, strontiumoxide, magnesium oxide, zinc oxide, calcium oxide, yttrium oxide,titanium oxide, niobium oxide, tantalum oxide, lanthanum oxide andmixtures thereof, and 0 to 10% by weight of colourants.
 6. A method forthe production of a lithium disilicate glass-ceramic according to claim1, in which a) an initial glass is produced which comprises thecomponents of the glass-ceramic, b) the initial glass is subjected to afirst heat treatment in order to produce a glass-ceramic which haslithium metasilicate as main crystal phase, c) the glass-ceramic of b)is subjected to a second heat treatment in which the lithiummetasilicate is converted with SiO₂ from the glass phase into lithiumdisilicate and subsequently lithium disilicate is present as maincrystal phase.
 7. The method according to claim 6, wherein the firstheat treatment is effected at a temperature of 620° C. to 800° C. over aperiod of time of 1 to 200 min.
 8. The method according to claim 7,wherein the second heat treatment is effected at a temperature of 800°C. to 1,040° C. over a period of time of 5 to 200 min. 9-10. (canceled)11. The lithium disilicate glass-ceramic according to claim 3, whereinthe rare earth metals are selected from the group consisting ofneodymium, praseodymium, samarium and europium.
 12. The method accordingto claim 7, wherein the first heat treatment is effected at atemperature of 650° C. to 750° C. over a period of time of 10 to 60 min.13. The method according to claim 8, wherein the second heat treatmentis effected at a temperature of 800° C. to 870° C. over a period of timeof 5 to 30 min.
 14. A dental material comprising the lithium disilicateglass-ceramic according to claim
 1. 15. A shaped dental productcomprising the lithium disilicate glass-ceramic according to claim 1.16. The shaped dental product according claim 15, which is an inlay, anonlay, a bridge, a pin construction, a veneer, or a crown.